Conventionally, there is known a differential unit for absorbing a difference in rotational speed between left and right wheels. The differential unit mainly includes a differential case, a pair of pinion gears which rotate together with the differential case (revolution), and a pair of side gears engaged with the pinion gears. The differential unit absorbs the difference in rotational speed between the left and right wheels by utilizing a fact that rotational speeds of the respective side gears change when the pinion gears rotate (rotation).
There is a differential unit that includes a spring between a differential case and a side gear, wherein the side gear is biased (see, for example, Patent Literature 1). Such a differential unit has an advantage that when the side gear hits the differential case, a contraction of the spring enables impact forces to be attenuated. However, since the contraction of the spring allows the side gear to move, it is required to separately provide a process of measuring the moving range of the side gear and determining whether the moving range of the side gear that has been measured is appropriate.
In the above process, a moving range measurement device capable of measuring the moving range of the side gear is used. The moving range measurement device includes a mechanism that comes close to the side gear in a direction vertical to a central axis direction of the side gear and presses the side gear in a direction parallel to the central axis direction. However, there is a problem that such a moving range measurement device is expensive since it includes a complicated mechanism including a plurality of actuators. Another problem is that, since it is required to insert a part of the mechanism into the differential case and press the side gear in the direction perpendicular to the direction in which the mechanism is inserted into the differential case, it is difficult to measure the moving range of the side gear.